Is your quantum simulator reliable? Scientists at MIT Disclose a New Quantum Phenomenon

 By Jennier Cho, Massachusetts University of Technology 

February 12, 2023

A recent innovation provides a way to verify the accuracy of research looking into the odd behaviour of atomic-scale systems.

Atomic scale physics becomes bizarre. To discover, exploit, and regulate these peculiar quantum phenomena, scientists are utilising quantum analogue simulators, which are laboratory experiments that entail cooling a large number of atoms to very low temperatures and observing them with perfectly calibrated lasers and magnets.

The objective is that any new knowledge gleaned from quantum simulators would serve as a template for creating novel exotic materials, more intelligent and effective electronics, and useful quantum computers. However, scientists must first have faith in quantum simulators in order to benefit from their discoveries.

They must make sure that their quantum device accurately captures quantum phenomena and has "high fidelity." Researchers might assume a quantum impact when there is none, for example, if an atom system is easily affected by outside noise. But until now, there hasn't been a trustworthy way to describe the quality of quantum analogue simulators.

In a paper that was recently published in Nature, physicists from MIT and Caltech describe a novel quantum phenomenon: they discovered that the quantum fluctuations of atoms exhibit some randomness, but this unpredictable behaviour follows a consistent, predictable pattern. It may seem contradictory to say that behaviour can be unpredictable and predictable. However, the group found that some random oscillations can in fact adhere to a recognisable statistical pattern.

Additionally, the researchers have developed a method to assess the authenticity of a quantum analogue simulator using this quantum randomness. They demonstrated through theory and experimentation that it was possible to evaluate a quantum simulator's random fluctuations in order to ascertain its accuracy.

The group created a brand-new benchmarking technique that can be used to assess the authenticity of current quantum analogue simulators using the quantum fluctuations they exhibit. The protocol might hasten the creation of novel exotic materials and quantum computing devices.

According to study co-author Soonwon Choi, an assistant professor of physics at MIT, "our work might allow characterising many existing quantum devices with very high precision." It also implies that the chaotic quantum systems' randomness is caused by deeper theoretical structures than previously believed.

Daniel Mark, an MIT graduate student, and associates from Caltech, Harvard University, the University of Illinois at Urbana-Champaign, and the University of California at Berkeley are the study's authors.

Random development

The 2019 development by Google, where scientists had created a digital quantum computer called "Sycamore," which could do a particular computation quicker than a classical computer, served as the impetus for the new study.

A quantum computer's "qubits" can exist in a superposition of several states as opposed to the "bits" of a classical computer, which can only exist as a 0 or a 1. Theoretically, special algorithms that handle challenging problems faster than any conventional computer can be executed when several qubits interact.

The Google scientists created a system of superconducting loops that behaved as 53 qubits, and they demonstrated that the "computer" was capable of doing a certain computation that would typically be too challenging for even the world's fastest supercomputer to complete.

Google also demonstrated its ability to measure the system's integrity. They determined the correctness of the system by randomly altering the states of each individual qubit and contrasting the 53 total qubit states with what the laws of quantum physics predict.

Choi and his colleagues questioned whether they might evaluate the fidelity of quantum analogue simulators using a similar, randomised method. But there was one obstacle they would have to overcome: Individual atoms and other qubits in analogue simulators are exceedingly difficult to manipulate and hence cannot be controlled arbitrarily, in contrast to Google's digital quantum system.

However, Choi discovered through some theoretical modelling that allowing the qubits to grow organically might replicate the system's overall effects of individually manipulating qubits in an analogue quantum simulator.

We discovered that we don't need to build this arbitrary behaviour, claims Choi. "We can just let the natural dynamics of quantum simulators evolve with no fine-tuning, and the result would lead to a similar pattern of randomness resulting from chaos," said the researcher.

Creating trust

Imagine a system with five qubits as an incredibly simple example. Until a measurement is done, each qubit can exist concurrently as a 0 or a 1, after which the qubits fall into one of the two states. The qubits can take on one of 32 distinct configurations with any given measurement, such as 0-0-0-0-0, 0-0-0-0-1, and so forth.

People assume that the probability distribution for these 32 configurations to occur will follow the predictions of statistical physics, as Choi explains. "We demonstrate that they agree on average, but there are variations and oscillations that reveal a hitherto unknown universal randomness." And the unpredictability appears to be identical to what you would get if you performed Google's random operations.

The researchers reasoned that they could compare the projected results with the simulator's real results if they could create a numerical simulation that accurately captures the dynamics and universal random fluctuations of a quantum simulator. The quantum simulator needs to be more precise the closer the two are together.

Choi collaborated with Caltech experimenters to create a 25-atom quantum analogue simulator to test this theory. The experiment's atoms were collectively excited by a laser before the scientists let the qubits interact and evolve organically over time. Over the course of several runs, they measured each qubit's state, amassing a total of 10,000 measurements.

The quantum dynamics of the experiment were also represented numerically by Choi and colleagues, who also included an equation they deduced to forecast the expected universal, random fluctuations. The researchers then compared their experimental results with those predicted by the model, and they discovered a very close match—strong evidence that this specific simulator can be relied upon to accurately portray pure, quantum mechanical behaviour.

More generally, the results show a novel method for characterising virtually every quantum analogue simulator already in existence.